Switching power supply circuit
A switching power supply circuit includes a rectifier inputting and rectifying an alternating-current voltage to produce a rectified voltage; a smoothing unit smoothing the rectified voltage; a switching unit including a switching element operable to perform a switching operation; a switching drive unit operable to drive the switching element; an isolation converter transformer including primary and secondary windings with a coupling coefficient to obtain adequate zero voltage switching in an intermediate load condition; a primary-side parallel resonant circuit including a leakage inductance component and a capacitance of a primary-side parallel resonant capacitor to make the switching unit exhibit voltage resonant operation; a secondary-side series resonant circuit including a leakage inductance component and a capacitance of a secondary-side series resonant capacitor; a secondary-side direct-current output voltage producing unit producing a secondary-side direct-current output voltage by a rectifying operation; a controller stabilizing the level of the secondary-side direct-current output voltage; and a power factor improving unit intermittently blocking a rectified current due to superposition of a switching output on the output of the rectifier.
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The present application claims priority from Japanese Patent Application Nos. JP 2005-192425 filed on Jun. 30, 2005, JP 2005-238473 filed on Aug. 19, 2005, JP 2005-196474 filed on Jul. 5, 2005, and JP 2005-196475 filed on Jul. 5, 2005, the disclosures of which are hereby incorporated by reference herein.
BACKGROUND OF THE INVENTIONThe present invention relates to a switching power supply circuit serving as a power supply included in various electronic apparatuses.
Due to recent development of switching elements that can withstand comparatively large current and voltage of a high frequency, major part of power supply circuits that rectify a commercial AC voltage to obtain a desired DC voltage employs a switching system.
The switching power supply circuits allow miniaturization of transformers and other devices through enhancement of a switching frequency, and are used as a high-power DC-DC converter for a power supply in various electronic apparatuses.
Typically rectifying of a commercial AC voltage leads to a distorted waveform of the current that flows to a smoothing circuit, and therefore a power factor, which indicates the use efficiency of the power supply, is problematically decreased.
Furthermore, there is a need for measures for suppressing harmonic waves arising due to the distorted current waveform.
Therefore, a method employing a so-called active filter has been known as a related technique for improving the power factor (e.g., refer to Japanese Patent Laid-Open No. Hei 6-327246, (
The basic configuration of such an active filter is illustrated in
Referring to
As the configuration for power factor improvement, an inductor L, a fast-recovery diode D, a resistor Ri, a switching element Q, and a multiplier 111 are provided as shown in the drawing.
The inductor L and the diode D are connected in series to each other between the positive output terminal of the bridge rectifier circuit Di and the positive electrode of the output capacitor Cout.
The resistor Ri is provided between the negative output terminal of the bridge rectifier circuit Di (primary-side ground) and the negative electrode of the output capacitor Cout.
A MOS-FET is chosen as the switching element Q in this circuit, and the switching element Q is provided between the connecting node of the inductor L and the diode D, and the primary-side ground.
Connected to the multiplier 111 are a current detecting line LI and a waveform input line Lw as a feedforward circuit, and a voltage detecting line Lv as a feedback circuit.
The multiplier 111 detects the level of the rectified current flowing from the negative output terminal of the bridge rectifier circuit Di, input through the current detecting line LI.
In addition, the multiplier 111 also detects the waveform of the rectified voltage from the positive output terminal of the bridge rectifier circuit Di, input through the waveform input line Lw. That is, the multiplier 111 detects the absolute values of the AC input voltage from the commercial alternating-current power supply line AC.
Furthermore, the multiplier ill detects a variation difference of the DC input voltage based on the DC voltage Vout across the output capacitor Cout input through the voltage detecting line Lv.
The multiplier 111 outputs a drive signal for driving the switching element Q.
The current detecting line LI supplies the multiplier 111 with the rectified current flowing through the negative output terminal of the bridge rectifier circuit Di. The multiplier 111 detects the level of the rectified current input from the current detecting line LI. Furthermore, the multiplier 111 detects a variation difference of the DC input voltage based on the DC voltage Vout across the output capacitor Cout input through the voltage detecting line Lv. In addition, the multiplier 111 also detects the waveform of the rectified voltage from the positive output terminal of the bridge rectifier circuit Di, input through the waveform input line Lw. That is, the multiplier 111 detects the absolute values of the AC input voltage.
The multiplier 111 initially multiplies the rectified current level detected through the current detecting line LI described above and the variation difference of the DC input voltage detected through the voltage detecting line Lv together. Subsequently, the multiplier 111 produces from the multiplication result and the waveform of the AC input voltage detected through the waveform input line Lw, a current command value with the same waveform as that of the AC input voltage VAC.
Moreover, the multiplier 111 compares the current command value with the actual AC input current level (detected based on the input from the current detecting line LI), and implements pulse width modulation (PWM) control for a PWM signal according to the difference therebetween, to produce a drive signal based on the PWM signal. The switching element Q is switch-driven by the drive signal. As a result, the AC input current is controlled so as to have the same waveform as that of the AC input voltage, so that the power factor is improved to almost 1. In addition, the current command value produced by the multiplier is controlled so that the amplitude thereof varies depending upon the variation difference of the rectified and smoothed voltage. Therefore, variation in the rectified and smoothed voltage is also suppressed.
The charging/discharging current Ichg has almost the same waveform as that of the second harmonic of the AC input voltage, unlike the input current Iin. Due to the energy flow from and to the output capacitor Cout, a ripple voltage Vdc arises in the second harmonic component of the AC input voltage as shown in
A switching preregulator 115 is provided between the positive output terminal of the bridge rectifier circuit Di and the positive electrode of the output capacitor Cout. The switching preregulator 115 is equivalent to the part formed of the switching element Q, the inductor L and the diode D in
The control circuitry including the multiplier 111 additionally includes a voltage error amplifier 112, a divider 113, and a squarer 114.
In the voltage error amplifier 112, the DC voltage Vout across the output capacitor Cout is divided by voltage-dividing resistors Rvo-Rvd, followed by being input to the non-inverting input of an operational amplifier 112a. A reference voltage Vref is input to the inverting input of the operational amplifier 112a. The operational amplifier 112a amplifies the voltage having the level corresponding to the error between the reference voltage Vref and the divided DC voltage Vout by the gain determined dependent upon a feedback resistor Rv1 and a capacitor Cv1, and outputs the amplified voltage as an error output voltage Vvea to the divider 113.
A so-called feedforward voltage Vff is input to the squarer 114. This feedforward voltage Vff is an output (average input voltage) resulting from averaging of the input voltage Vin by an averaging circuit 116 (Rf11, Rf12, Rf13, Cf11, Cf12). The squarer 114 squares the feedforward voltage Vff and outputs the resultant voltage to the divider 113.
The divider 113 divides the error output voltage Vvea from the voltage error amplifier 112 by the square of the average input voltage output from the squarer 114, and outputs a signal as the division result to the multiplier 111.
That is, a voltage loop is formed of the system including the squarer 114, the divider 113 and the multiplier 111. The error output voltage Vvea output from the voltage error amplifier 112 is divided by the square of the average input voltage (Vff) before being multiplied by a rectified input signal Iac in the multiplier 111. Due to this circuitry, the gain of the voltage loop is not changed but kept constant as the square of the average input voltage (Vff). The average input voltage (Vff) has the function of an open loop improvement sent in the forward direction in the voltage loop.
Input to the multiplier 111 are the output resulting from the division of the error output voltage Vvea by the divider 113 and the rectified output (Iac) from the positive output terminal of the bridge rectifier circuit Di (rectified output line) via a resistor Rvac. In
Therefore, the current programming signal has the waveform of the average amplitude for controlling the input voltage and the output voltage. The active filter controls not only the output voltage Vout but also the input voltage Vin. Since it can be said that the current loop in the feedforward circuit is programmed based on the rectification line voltage, the input to a subsequent-stage converter (load 110) is resistive input.
In the power supply circuit in
The rectified output line from the bridge rectifier circuit Di is connected to a normal mode noise filter 125 obtained by coupling one choke coil LN and two filter capacitors (film capacitors) CN, CN to each other as shown in the drawing.
The positive output terminal of the bridge rectifier circuit Di is coupled to the positive electrode of a smoothing capacitor Ci via the series connection of the choke coil LN, an inductor Lpc in a power choke coil PCC, and a fast-recovery rectifier diode D20. This smoothing capacitor Ci is equivalent to the output capacitor Cout in
The rectifier diode D20 is connected in parallel to an RC snubber circuit formed by the series connection of a capacitor Csn and a resistor Rsn.
A switching element Q3 is equivalent to the switching element Q in
A MOS-FET is chosen as the switching element Q3.
A power-factor/output-voltage control IC 120 is an integrated circuit (IC) for controlling the operation of the active filter that improves the power factor so that the power factor is brought close to 1.
The power-factor/output-voltage control IC 120 includes a multiplier, a divider, a voltage error amplifier, a PWM control circuit, and a drive circuit that outputs a drive signal for switch-driving the switching element. Circuitry equivalent to the multiplier 111, the voltage error amplifier 112, the divider 113, and the squarer 114 shown in
A feedback circuit is formed so that a voltage resulting from division of the voltage across the smoothing capacitor Ci (rectified and smoothed voltage Ei) by voltage-dividing resistors R5 and R6 is input to a terminal Ti of the power-factor/output-voltage control IC 120.
Furthermore, as a feedforward circuit, a series circuit of voltage-dividing resistors R1 and R2 is provided between the positive output terminal of the bridge rectifier circuit Di and the primary-side ground. The connecting node between the voltage-dividing resistors R1 and R2 is connected to a terminal T5. Thus, the rectified output from the bridge rectifier circuit Di is input to the terminal T5. In this manner, the feedforward circuit is formed as the line equivalent to the current detecting line LI in
A terminal T4 is supplied with an operating supply voltage for the power-factor/output-voltage control IC 120. Specifically, an alternating voltage is excited in a winding N5 that is transformer-coupled to the inductor Lpc in the power choke coil PCC. The excited alternating voltage is converted to a low DC voltage by a half-wave rectifier circuit formed of a diode D11 and a capacitor C11, and this DC voltage is input to the terminal T4.
The terminal T4 is also coupled to the positive output terminal of the bridge rectifier circuit Di via a start-up resistor Rs. When the commercial alternating-current power supply AC is powered up and the power supply circuit is activated, the rectified output obtained from the positive output terminal of the bridge rectifier circuit Di is supplied via the start-up resistor Rs to the terminal T4. The power-factor/output-voltage control IC 120 uses the thus supplied rectified output as its start-up supply voltage to thereby start the operation.
From a terminal T3, a drive signal (gate voltage) for driving the switching element is output to the gate of the switching element Q3.
The switching element Q3 implements switching operation in response to the applied drive signal.
A resistor R3 is provided between the source of the switching element Q3 and the primary-side ground, and the connecting node between the source of the switching element Q3 and the resistor R3 is connected to a terminal T2. The resistor R3 is to detect an overcurrent flowing through the switching element Q3 as a voltage. When a voltage with the level corresponding to an overcurrent is detected through the terminal T2, the power-factor/output-voltage control IC 120 implements protective operation, such as operation of stopping the switching operation of the switching element Q3.
The switch-driving of the switching element Q3 is implemented by a drive signal based on PWM control so that the conduction angle of the rectified output current becomes almost the same as that of the rectified output voltage waveform as described above with
The power factor improvement operation of an actual active filter with the above-described configuration will be described below with reference to
As is apparent from a comparison among
According to this switching operation, when the load is a light load, the current I1 flows with a discontinuous mode in which the periods when the current I1 does not flow exist. Under an intermediate load condition, the current I1 flows with a critical mode. In a heavy load range, it flows with a continuous mode. This operation mode transition of the current I1 depends on selection of the inductor Lpc. In order to allow an actual circuit to operate compatibly with commercial AC supply inputs of the AC 100 V-system, the inductor Lpc having an inductance of 140 μH is chosen for example. By thus adjusting the current I1 flowing via the inductor Lpc to the smoothing capacitor Ci depending upon the load condition, the rectified and smoothed voltage Ei can be stabilized against level variation of the AC input voltage VAC and load variation. In the circuit of
As shown in
In addition to the power factor improvement, stabilization of the rectified and smoothed voltage Ei at an average value of 380 V is also indicated. The waveform of an actual rectified and smoothed voltage Ei includes ripple fluctuation of 10 Vp-p about 380 V as shown in
The current resonant converter, which is at the subsequent stage of the active filter, is fed with the rectified and smoothed voltage Ei as a DC input voltage and implements switching operation for power conversion. The current resonant converter includes two-transistor switching elements Q1 and Q2 as shown in the drawing. The switching elements Q1 and Q2 are half-bridge connected to each other in such a manner that the switching element Q1 is on the high side and the switching element Q2 is on the low side. The switching elements Q1 and Q2 are connected in parallel to the rectified and smoothed voltage Ei (DC input voltage). That is, a current resonant converter based on a half-bridge connection system is formed.
This current resonant converter is separately excited, and correspondingly MOS-FETs are used as the switching elements Q1 and Q2. Clamp diodes DD1 and DD2 are connected in parallel to these switching elements Q1 and Q2, respectively, which forms a switching circuit. These clamp diodes DD1 and DD2 form a path of a reverse-direction current when the switching elements Q1 and Q2 are in the off-state.
The switching elements Q1 and Q2 are switch-driven by an oscillation and drive circuit 2 at a requisite switching frequency so that they are alternately tuned on and off. The oscillation and drive circuit 2 operates to vary the switching frequency based on control executed by a control circuit 1 dependent upon the level of a secondary-side DC output voltage Eo, to thereby stabilize the secondary-side DC output voltage Eo.
An isolation converter transformer PIT is provided in order to transmit the switching output of the switching elements Q1 and Q2 from the primary side to the secondary side.
One end of a primary winding N1 of the isolation converter transformer PIT is coupled via a series resonant capacitor C1 to the connecting node (switching output node) between the switching elements Q1 and Q2, while the other end is connected to the primary-side ground. The capacitance of the series resonant capacitor C1 and the leakage inductance (L1) of the primary winding N1 form a series resonant circuit. This series resonant circuit is supplied with the switching output from the switching elements Q1 and Q2 to thereby implement resonant operation. Thus, current resonant operation is obtained as the operation of the switching circuit formed of the switching elements Q1 and Q2.
A secondary winding N2 is wound on the secondary side of the isolation converter transformer PIT.
The secondary winding N2 is provided with a center tap as shown in the drawing, and this center tap is connected to the secondary-side ground. Furthermore, rectifier diodes Do1 and Do2 and a smoothing capacitor Co are provided to form a double-wave rectifier circuit as shown in the drawing. Thus, the secondary-side DC output voltage Eo is obtained as the voltage across the smoothing capacitor Co. The secondary-side DC output voltage Eo is supplied to a load (not shown), and is branched and input to the control circuit 1 as a detected voltage therefore. The control circuit 1 supplies to the oscillation and drive circuit 2 a control signal dependent upon the level of the input secondary-side DC output voltage Eo. The oscillation and drive circuit 2 drives the switching elements Q1 and Q2 with varying the switching frequency so that the secondary-side DC output voltage Eo is stabilized according to the control signal. That is, stabilization based on a switching frequency control system is carried out.
Referring initially to
The following experimental results were obtained from an actual circuit under the load condition of the load power Po of 300 W: ηAC→DC was about 83.0% when the AC input voltage VAC was 100 V; and ηAC→DC was about 89.0% when it was 230 V.
Turning back to
The following experimental results were obtained from an actual circuit under the load condition of the load power Po of 300 W: the power factor PF was about 0.96 when the AC input voltage VAC was 100 V; and the power factor PF was about 0.94 when the AC input voltage VAC was 230 V.
As for the rectified and smoothed voltage Ei, constancy is exhibited both against variation of the load power Po and against variation of the AC input voltage VAC as shown in
As is apparent from the above description, the power supply circuit in
However, the power supply circuit with the configuration of
Specifically, the power conversion efficiency of the power supply circuit in
That is, the total power conversion efficiency of the circuit in
According to an experiment on the circuit of
Accordingly, also as described above, the total AC to DC power conversion efficiency of the circuit in
Furthermore, since the active filter circuit implements hard-switching operation, the degree of noise generation is significantly high, which needs measures for suppression of comparatively severe noise.
Therefore, in the circuit shown in
In addition, the rectified output line is provided with a normal mode noise filter made up of one choke coil LN and two filter capacitors CN. Moreover, an RC snubber circuit is provided for the fast-recovery diode D20 for rectification.
In this manner, an actual circuit needs to be provided with anti-noise measures formed with a significantly large number of components, which results in a cost increase and a larger mounting area of the power supply circuit board.
Furthermore, the switching frequency of the switching element Q3 operated by the power-factor/output-voltage control IC 120 as a general-purpose IC is fixed at 60 kHz, while the switching frequency of the subsequent-stage current resonant converter varies in the range from 80 kHz to 200 kHz. Since the switching timings of the both switching elements are thus independent of each other, the primary-side ground potentials interfere with each other due to the switching operation of the both elements and therefore become unstable, which readily causes abnormal oscillation for example. This causes problems such as difficulty in circuit design and deterioration of the reliability.
SUMMARY OF THE INVENTIONIn consideration of the above-described problems, one embodiment of the present invention provides a switching power supply circuit having the following configuration.
Specifically, the switching power supply circuit includes a rectifying and smoothing unit supplied with an AC voltage and produces a rectified and smoothed voltage; a switching unit including a switching element supplied with the rectified and smoothed voltage as a DC input voltage and performing a switching operation, and a switching drive unit that drives the switching element.
The switching power supply circuit also includes an isolation converter transformer having a primary winding on a primary side and a secondary winding on a secondary side. The primary winding is supplied with a switching output obtained from the switching operation of the switching element. The switching output obtained in the primary winding induces an alternating voltage in the secondary winding. The isolation converter transformer has a structure producing a coupling coefficient between the primary side and the secondary side of the isolation converter transformer such that adequate zero voltage switching is ensured even in a range of certain load conditions regarded as an intermediate load condition.
The switching power supply circuit further includes a primary-side parallel resonant circuit including a leakage inductance component of the primary winding of the isolation converter transformer and a capacitance of a primary-side parallel resonant capacitor. The primary-side parallel resonant circuit makes the switching unit operate on a voltage resonant basis.
Moreover, the switching power supply circuit also includes a secondary-side series resonant circuit including a leakage inductance component of the secondary winding and a capacitance of a secondary-side series resonant capacitor connected in series with the secondary winding.
Furthermore, the switching power supply circuit also includes a secondary-side DC output voltage producing unit that is supplied with the alternating voltage induced in the secondary winding of the isolation converter transformer, and that performs a rectifying operation to thereby produce a secondary-side DC output voltage.
In addition, the switching power supply circuit further includes a power factor improvement unit that implements at least an operation of feeding back the switching output, in which a switching output current that arises in the primary winding due to the switching operation of the switching unit is regenerated as power and is fed back to the smoothing capacitor forming the rectifying and smoothing unit. The power factor improvement unit intermittently blocks, by use of a power factor improvement switching element, a rectified current that arises from the rectifying operation of the rectifying and smoothing unit in response to the fed back switching output.
The term coupling coefficient in the present specification indicates the degree of electromagnetic coupling. A coefficient of 1 corresponds to the highest coupling degree, while a coefficient of 0 corresponds to the lowest coupling degree (non-coupled state).
The power supply circuit with the above-described configuration according to one embodiment of the present invention employs a basic configuration as a switching converter that includes a primary-side voltage resonant converter and a secondary-side series resonant circuit. In addition, in order to achieve power factor improvement, a current (switching output current) obtained in the primary winding of the isolation converter transformer in response to the switching output of the switching element is regenerated as power and is fed back to the rectified current path (smoothing capacitor for producing a rectified and smoothed voltage), and the power factor improvement switching element implements switching for the rectified current due to the fed-back voltage.
This configuration eliminates the need to provide an active filter for stabilizing a DC input voltage to a switching converter in order to construct a power supply circuit including a power factor improvement circuit.
As described above, a switching power supply circuit according to one embodiment of the present invention can serve as a switching power supply circuit having a power factor improvement function without an active filter. The absence of the active filter enhances the power conversion efficiency characteristic of the switching power supply circuit. This leads to, e.g., elimination or miniaturization of a heat sink and so on. In addition, the number of components is greatly reduced compared with a configuration including an active filter, which allows a reduction in the size and weight of the circuit and a cost decrease. Moreover, contrary to the hard-switching operation of an active filter, a switching converter of one embodiment of the invention implements a soft-switching operation since it is based on a voltage resonant converter. This significantly reduces switching noise and thus eliminates the need to reinforce noise filters, which also contributes to the size and weight reduction and cost decrease.
Furthermore, according to the configuration of one embodiment of the invention, simultaneous operation with different switching frequencies is absent, and thus the problem of interference between the ground potentials of the primary and secondary sides is eliminated. Therefore, the ground potentials are stabilized, which enhances the reliability and facilitates pattern design of a circuit board.
BRIEF DESCRIPTION OF THE DRAWINGS
In the switching power supply circuit in
An AC input voltage VAC is rectified by a bridge rectifier circuit Di that is formed by bridge-connecting four low-speed rectifier elements (diodes), and the rectified output is charged in a smoothing capacitor Ci. Thus, a rectified and smoothed voltage Ei is obtained as the voltage across the smoothing capacitor Ci. The rectified and smoothed voltage Ei has the same level as that of the AC input voltage VAC. The rectified and smoothed voltage Ei serves as a DC input voltage for the switching converter at the subsequent stage.
In the present embodiment, a power factor improvement circuit 10 is provided to intervene on the line between the positive output terminal of the bridge rectifier circuit Di and the positive electrode of the smoothing capacitor Ci. The configuration and operation of the power factor improvement circuit 10 will be described later.
In
Applied to the gate of the switching element Q1 is a switching drive signal (voltage) output from the oscillation and drive circuit 2.
The drain of the switching element Q1 is coupled to the winding-start end of a primary winding N1 in an isolation converter transformer PIT to be described later. The winding-finish end of the primary winding N1 is coupled to the positive electrode of the smoothing capacitor Ci via a high-frequency choke coil L10 to be described later in the power factor improvement circuit 10. That is, in the present embodiment, the DC input voltage (Ei) is supplied to the switching element Q1 via the primary winding N1 connected in series thereto. The source of the switching element Q1 is coupled to the primary-side ground.
Since a MOS-FET is chosen as the switching element Q1, the switching element Q1 incorporates a body diode DD so that the diode DD is connected in parallel to the channel between the source and drain of the switching element Q1 as shown in the drawing. The anode of the body diode DD is connected to the source of the switching element Q1, and the cathode thereof is connected to the drain of the switching element Q1. The body diode DD forms the path of a switching current in the reverse direction, arising due to the ON/OFF operation (switching operation) of the switching element Q1.
In addition, a primary-side parallel resonant capacitor Cr is connected in parallel to the channel between the drain and source of the switching element Q1.
The capacitance of the primary-side parallel resonant capacitor Cr and the leakage inductance L1 of the primary winding N1 in the isolation converter transformer PIT form a primary-side parallel resonant circuit (voltage resonant circuit) for the switching current flowing through the switching element Q1. The resonant operation of this primary-side parallel resonant circuit offers voltage resonant operation as the switching operation of the switching element Q1. Due to this operation, a voltage V1 across the switching element Q1 (voltage between the drain and source) has a sinusoidal resonant pulse voltage waveform during the OFF period of the switching element Q1.
In order to drive the switching element Q1 by separate excitation for example, the oscillation and drive circuit 2 produces a drive signal that is a gate voltage for switch-driving the MOS-FET based on an oscillation circuit and an oscillation signal obtained due to the oscillation circuit, and applies the drive signal to the gate of the switching element Q1. Thus, the switching element Q1 continuously implements ON/OFF operation with a switching frequency dependent upon the cycle of the drive signal. That is, the switching element Q1 implements switching operation.
The isolation converter transformer PIT transmits the switching output from the primary-side switching converter to the secondary side, with isolating the primary side from the secondary side in terms of DC transmission therebetween.
As shown in
Furthermore, a bobbin B is provided that is formed of resin or the like and has such a divided shape that winding parts on the primary side and secondary side are independent of each other. The primary winding N1 is wound around one winding part of the bobbin B. The secondary winding N2 is wound around the other winding part.
The bobbin B around which the primary-side and secondary-side windings have been thus wound is fitted to the EE-shaped core (CR1, CR2), which results in the state in which the primary-side and secondary-side windings on different winding regions are wound around the center leg of the EE-shaped core. In this manner, the entire structure of the isolation converter transformer PIT is completed.
In the center leg of the EE-shaped core, a gap G having a gap length of e.g., about 1.6 mm or more is formed as shown in the drawing. Thus, a loose coupling state in which a coupling coefficient k is e.g., about 0.75 is obtained. That is, the degree of loose coupling is higher in the isolation converter transformer PIT in
As described above, one end of the primary winding N1 in the isolation converter transformer PIT is connected to the drain of the switching element Q1. Thus, the switching output from the switching element Q1 is transmitted to the primary winding N1, and therefore an alternating voltage arises in the primary winding N1.
On the secondary side of the isolation converter transformer PIT, an alternating voltage induced by the primary winding N1 is generated in the secondary winding N2.
Connected in series to the secondary winding N2 is a secondary-side series resonant capacitor C2. Thus, the leakage inductance L2 of the secondary winding N2 and the capacitance of the secondary-side series resonant capacitor C2 form a secondary-side series resonant circuit. This secondary-side series resonant circuit implements resonant operation in association with the rectifying operation of a secondary-side rectifier circuit to be described later. Thus, a secondary winding current flowing through the secondary winding N2 has a sinusoidal waveform. That is, current resonant operation is achieved on the secondary side.
The secondary-side rectifier circuit in the present embodiment is formed as a voltage-doubler half-wave rectifier circuit by coupling two rectifier diodes Do1 and Do2 and one smoothing capacitor Co to the secondary winding N2, to which the secondary-side series resonant capacitor C2 is connected in series as described above. The connecting structure of the voltage-doubler half-wave rectifier circuit is as follows. The winding-finish end of the secondary winding N2 is coupled via the secondary-side series resonant capacitor C2 to the anode of the rectifier diode Do1 and the cathode of the rectifier diode Do2. The cathode of the rectifier diode Do1 is connected to the positive electrode of the smoothing capacitor Co. Connected to the secondary-side ground are the winding-start end of the secondary winding N2, the anode of the rectifier diode Do2, and the negative electrode of the smoothing capacitor Co.
The rectifying operation of the thus formed voltage-doubler half-wave rectifier circuit is as follows.
In the periods of half cycles corresponding to one polarity of an alternating voltage obtained in the secondary winding, a forward voltage is applied to the rectifier diode Do2 and thus the rectifier diode Do2 conducts, which offers operation of charging the secondary-side series resonant capacitor C2 with the rectified current. Thus, generated across the secondary-side series resonant capacitor C2 is the voltage with the same level as that of the alternating voltage induced in the secondary winding N2. In the periods of half cycles corresponding to the other polarity of the alternating voltage obtained in the secondary winding, the rectifier diode Do1 is provided with a forward voltage and thus conducts. The smoothing capacitor Co is then charged by the potential resulting from superposition of the secondary winding voltage and the voltage across the secondary-side series resonant capacitor C2.
Thus, generated across the smoothing capacitor Co is a secondary-side DC output voltage Eo having the level equal to twice the level of the alternating voltage induced in the secondary winding N2. In this rectifying operation, the charging of the smoothing capacitor Co is implemented in the periods of half cycles of one polarity of the alternating voltage excited in the secondary winding N2. That is, rectifying operation as voltage-doubling half-wave rectifying is achieved.
This secondary-side DC output voltage Eo is supplied to a load. In addition, the voltage Eo is branched and output to a control circuit 1 as a detected voltage.
The control circuit 1 supplies to the oscillation and drive circuit 2 a detection output dependent upon a level change of the input secondary-side DC output voltage Eo. The oscillation and drive circuit 2 drives the switching element Q1, with varying the switching frequency or the time ratio between the periods TON and TOFF (conduction angle) within one switching cycle according to the detection output input from the control circuit 1. This operation serves as constant-voltage control operation for the secondary-side DC output voltage.
In the basic constant-voltage control operation of a voltage resonant converter, the ON period TON of the switching element Q1 is varied while the OFF period TOFF thereof is kept constant, to thereby vary the switching frequency. However, if the voltage resonant converter includes a secondary-side series resonant circuit like the present embodiment, another constant-voltage control operation as PWM control operation in which the conduction angle in one switching cycle is controlled is also implemented as described above. That is, as a whole of the constant-voltage control operation, complex control including the switching frequency control and PWM control is conducted.
This variation control of switching frequency and conduction angle of the switching element Q1 leads to changes of resonant impedances of the primary and secondary sides and the power transmission effective period, in the power supply circuit. These changes result in a change of the amount of power transmitted from the primary winding N1 to the secondary winding N2 in the isolation converter transformer PIT, and in a change of the amount of power that should be supplied from the secondary-side rectifier circuit to a load. Thus, the level of the secondary-side DC output voltage Eo is controlled so that the level variation thereof is cancelled. That is, stabilization of the secondary-side DC output voltage Eo is provided.
A description will be made below on the power factor improvement circuit 10.
The power factor improvement circuit 10 is provided in the middle of the rectified current path in a rectifying and smoothing circuit for producing a DC input voltage (Ei) from the AC input voltage VAC, and employs a configuration for improving the power factor by use of a power regenerative system based on magnetic coupling.
In the power factor improvement circuit 10, the anode of a fast-recovery switching diode D1 (power factor improvement switching element) is connected to the positive output terminal of the bridge rectifier circuit Di. The cathode of the switching diode D1 is connected in series to the high-frequency choke coil L10, and is coupled to the positive electrode of the smoothing capacitor Ci via the choke coil L10. That is, a series circuit of the switching diode D1 (anode to cathode) and the high-frequency choke coil L10 is provided between the positive output terminal of the bridge rectifier circuit Di and the positive electrode of the smoothing capacitor Ci.
Furthermore, a filter capacitor CN is connected in parallel to the series circuit of the switching diode D1 and the high-frequency choke coil L10. The filter capacitor CN is to suppress normal mode noise.
In addition, one end of the primary winding N1 (winding-finish end, in this embodiment) is coupled to the connecting node between the high-frequency choke coil L10 and the cathode of the switching diode D1. This connecting node serves as a switching output feedback node in this embodiment.
This circuit configuration of the power factor improvement circuit 10 offers the following operation. Specifically, a switching output current that arises in the primary winding N1 in response to the switching operation of the switching element Q1 (primary-side parallel resonant current arising due to the resonant operation of the primary-side parallel resonant circuit) is regenerated as power and is fed back to the smoothing capacitor Ci through magnetic coupling obtained due to the high-frequency choke coil L10.
Thus, e.g., the voltage between the connecting node of the primary winding N1 and the power factor improvement circuit 10 and the primary-side ground has the waveform resulting from superposition of an alternating voltage component with the switching cycle on the rectified and smoothed voltage Ei. Due to application of this alternating voltage component superimposed on the rectified and smoothed voltage Ei to the fast-recovery switching diode D1, the diode D1 implements switching operation when the absolute value of positive/negative level of the AC input voltage VAC is larger than about half of its peak value for example, to thereby intermittently block the rectified current directed toward the smoothing capacitor Ci.
The conduction period of envelope of the thus flowing rectified current is such that the current flows also during the periods when the level of the rectified output voltage from the bridge rectifier circuit Di is lower than that of the voltage across the smoothing capacitor Ci. In addition, the conduction period of the AC input current IAC that flows based on the AC input voltage VAC substantially corresponds with the conduction period of this rectified current. Specifically, the conduction angle of the AC input current IAC is increased compared with a configuration including no power factor improvement circuit, and the waveform of the AC input current IAC is brought close to that of the AC input voltage VAC. That is, power factor improvement is achieved.
For an experiment on an actual power supply circuit with the above-described configuration of
The isolation converter transformer PIT was based on the structure shown in
For the high-frequency choke coil L10, an EE-22 core was chosen and a gap of 1.4 mm was formed, so that the inductance L10 was set to 27 μH.
As is well known, the above-described EER and EE cores are one of types and standards of product cores. It is known that the types also include an ER type. The sectional shapes of cores of all the EER, ER and EE types are an E-character shape or EE-character shape. Therefore, cores of all the EER, ER and EE types are treated as the E-shaped core and EE-shaped core in the present specification.
The capacitances of the primary-side parallel resonant capacitor Cr, the filter capacitor CN and the secondary-side series resonant capacitor C2 were chosen as follows: Cr=7500 pF, CN=1 μF, and C2=0.047 μF.
The allowable load power range was from the maximum load power Pomax of 300 W to the minimum load power Pomin of 0 W (no load). The rated level of the secondary-side DC output voltage Eo was 175 V.
The experimental results on the power supply circuit of
A current I1 is the current flowing from the switching diode D1 to the high-frequency choke coil L10 in the power factor improvement circuit 10. As shown in
A current I2 flowing into the smoothing capacitor Ci is an alternating current that has the switching cycle and flows with the illustrated waveform. The current I2 results from synthesis of the current I1 and the primary winding current obtained in the primary winding N1 and subsequent passage of the synthesized current through the high-frequency choke coil L10.
A voltage V2 is the voltage between the primary-side ground and the connecting node of the switching diode D1 and the high-frequency choke coil L10, which is the switching output feedback node. As shown in
A high-frequency signal is not superimposed on a voltage V3 of the output terminal of the bridge rectifier circuit Di because of the operation of the filter capacitor CN.
The AC input current IAC has the illustrated waveform, which includes half sinewaves that arise in the conducting periods of the current I1 and have opposite polarities corresponding to the polarity of the AC input voltage VAC. This waveform includes an increased conduction angle compared with e.g. the configuration obtained by eliminating the power factor improvement circuit 10 from the circuit of
In addition,
The rectified and smoothed voltage Ei, which is the DC input voltage for the switching converter, changes depending upon the AC input voltage VAC as shown in
The power factor PF, which depends on the operation of the power factor improvement circuit 10, has a tendency of increasing as the load becomes heavier as shown in
As for the AC to DC power conversion efficiency (ηAC→DC), as shown in
The following discussion is based on a comparison between the above-described power supply circuit according to the embodiment and the power supply circuit in
Initially, as is apparent also from the description of the experimental results shown in
This improvement is due mainly to the configuration that includes a power factor improvement circuit based on a power regenerative system and therefore eliminates the need to include an active filter. That is, unlike the configuration with an active filter, the present embodiment does not involve lowering of the total efficiency due to multiplication of two power conversion efficiencies of the former and latter stages.
Furthermore, since the circuit in
Specifically, an active filter involves the formation of one converter, and hence is constructed of a large number of components such as one switching element and an IC for driving the switching element, as is apparent from the description for
On the contrary, it is sufficient for the power supply circuit in
Thus, the power supply circuit in
Moreover, in the power supply circuit in
Accordingly, even provision of e.g. a one-stage noise filter formed of a pair of common mode choke coils CMC and two across-line capacitors CL, shown in
This reduced number of components for noise filters also promotes a cost decrease of the power supply circuit and a reduction in the size and weight of the circuit board.
Furthermore, in the power supply circuit of
The power factor PF obtained in the power supply circuit in
As described above, the power supply circuit of the present embodiment shown in
The power supply circuit of the present embodiment includes on its primary side a single-ended voltage resonant converter as its basic configuration.
It is typically considered that a power supply circuit including a voltage resonant converter on its primary side may not be put into practical use as it stands, since it has a narrow allowable range of load power and may not maintain Zero Voltage Switching (ZVS) operation when the load is light. The present inventor therefore has made an experiment on a power supply circuit in which a secondary-side series resonant circuit is provided for a primary-side voltage resonant converter and a voltage-doubler half-wave rectifier circuit is formed as a secondary-side rectifier circuit. The power supply circuit as the experimental target had a circuit configuration obtained by eliminating the power factor improvement circuit 10 from the power supply circuit in
The isolation converter transformer PIT in this circuit basically had the same structure as that in
However, it has been confirmed that this power supply circuit as the experimental target involves abnormal operation when the load is an intermediate load. Specifically, when the load is an intermediate load, a current flows through the switching element Q1 in the positive direction (from the drain to the source) before the end of the OFF period (TOFF) of the switching element Q1, and thus ZVS operation fails to be achieved. Accordingly, it is still difficult to put the circuit into practical use, even with the configuration of the power supply circuit as the experimental target.
On the contrary, as for the power supply circuit of the present embodiment shown in
The elimination of the abnormal operation associated with an intermediate load is due mainly to the feature that the isolation converter transformer PIT has such a loose coupling state that the coupling coefficient k is smaller than 0.7 for example.
It has been confirmed that the above-described abnormal operation associated with an intermediate load readily arises if a voltage resonant converter includes a secondary-side series resonant circuit. Specifically, the abnormal operation is attributed to the interaction between the primary-side parallel resonant circuit that forms the voltage resonant converter and the secondary-side series resonant circuit, due to the simultaneous operation.
Accordingly, if, like the power supply circuit of the present embodiment, the coupling coefficient k of the isolation converter transformer PIT is set to a requisite value lower than values of related circuits, the above-described interaction between the primary-side parallel resonant circuit and secondary-side series resonant circuit is attenuated, which correspondingly eliminates the abnormal operation associated with an intermediate load.
Avoided in related voltage resonant converters is the enhancement of degree of loose coupling to such an extent that the coupling coefficient k becomes the same as that of the isolation converter transformer PIT of the present embodiment. This is because such loose coupling causes lowering of the power conversion efficiency due to an increase in loss of power transmission from the primary side to the secondary side.
However, the present embodiment has a favorable power conversion efficiency characteristic even in terms of a power supply circuit including a primary-side voltage resonant converter.
The achievement of high power conversion efficiency in the present embodiment is due to the following configuration.
It is known that a power supply circuit including a voltage resonant converter combined with a secondary-side series resonant circuit is originally advantageous for obtaining high power conversion efficiency. Furthermore, if a single-ended system is employed for a voltage resonant converter and therefore the number of switching elements is one, which is the bare minimum number, switching loss of the converter is smaller compared with configurations including plural switching elements, such as a half-bridge connection system, a full-bridge connection system, and a push-pull system. This reduced switching loss also leads to the enhancement of the power conversion efficiency.
In addition, the present embodiment eliminates the occurrence of abnormal operation associated with an intermediate load to thereby allow adequate ZVS operation as described above. In this abnormal operation phenomenon, the switching element Q1 is turned on and the positive switching current flows between the source and drain before the original turn-on timing of the switching element Q1. Such behavior of the switching current leads to an increase of switching loss. The present embodiment prevents the occurrence of this behavior of the switching current corresponding to the abnormal operation, and thus eliminates the increase of switching loss. This feature is also one factor in the enhancement of the power conversion efficiency.
Moreover, the present inventor has confirmed that varying the resonant frequencies fo1 and fo2 of the primary-side parallel resonant circuit and the secondary-side series resonant circuit, respectively, also leads to a change of the power conversion efficiency characteristic (ηAC→DC). Therefore, the power conversion efficiency in the power supply circuit of the present embodiment can be enhanced also by setting the resonant frequencies fo1 and fo2 of the primary-side parallel resonant circuit and the secondary-side series resonant circuit so that as favorable a power conversion efficiency characteristic as possible is achieved.
For example, it has been confirmed that when the relationship between the resonant frequencies fo1 and fo2 of the primary-side parallel resonant circuit and the secondary-side series resonant circuit is varied, the waveform of the switching current flowing through the switching element Q1 changes due to the influence of the resonant frequencies. As one example, depending on the relationship between the resonant frequencies fo1 and fo2 of the primary-side parallel resonant circuit and the secondary-side series resonant circuit, the switching current flowing through the switching element Q1 has a waveform in which the current peak appears before the turn-off timing of the switching element Q1 and the current level at the turn-off timing is lower than the peak level. This waveform includes the waveform component dependent upon the current resonant waveform obtained due to the resonant operation of the secondary-side series resonant circuit. If the level of the switching current at the turn-off timing is thus suppressed, the switching loss at the turn-off timing is correspondingly reduced, which enhances the power conversion efficiency.
As described above, in the present embodiment, the resonant frequency fo1 of the primary-side parallel resonant circuit and the resonant frequency fo2 of the secondary-side series resonant circuit are optimally set so that as high power conversion efficiency as possible is obtained, in consideration of the stability of operation of the power supply circuit.
Moreover, in the power supply circuit in
The power supply circuit in
The anode of the rectifier diode D1A is connected to the positive output terminal of a bridge rectifier circuit Di, and the cathode thereof is connected to the positive electrode of a smoothing capacitor Ci. Therefore, in the power factor improvement circuit 11, the diode D1A is connected in parallel to a series circuit of a switching diode D1 and a high-frequency choke coil L10. A filter capacitor CN in the present embodiment is connected in parallel to the series circuit of the switching diode D1 and the high-frequency choke coil L10, and in parallel to the rectifier diode D1A.
In the power supply circuit of the second embodiment with the thus formed power factor improvement circuit 11, a rectified current obtained as the rectified output from the positive output terminal of the bridge rectifier circuit Di flows with being branched into the series circuit of the switching diode D1 and the high-frequency choke coil L10, and into the rectifier diode D1A. Also in such operation, the basic operation of the power factor improvement circuit 11 is the same as that of the power factor improvement circuit 10. Specifically, an alternating voltage with the switching cycle is superimposed on the rectified current path by regenerating and feeding back the switching output current as power. This alternating voltage causes the switching diode D1 to implement switching for the rectified current and thus increase the conduction angle of the AC input current IAC, so that power factor improvement is achieved.
In addition, since the rectified current from the bridge rectifier circuit Di flows with being branched also into the rectifier diode D1A as described above, the amount of the rectified current flowing through the switching diode D1 is decreased. This reduces switching loss in the switching diode D1 and therefore enhances the power conversion efficiency. This advantage becomes more effective as the load becomes heavier and therefore the current flowing through the power supply circuit becomes larger in particular.
The power supply circuit in
The switching output feedback node in the present embodiment is the tap of the high-frequency choke coil L10 (connecting node between the choke coil winding portions L10A and L10B), and the winding-finish end of a primary winding N1 is connected to the tap. In addition, a filter capacitor CN is connected in parallel to a series circuit of the switching diode D1 and the high-frequency choke coil L10 (choke coil winding portions L10A and L10B).
In the thus formed power factor improvement circuit 12, the rectified current output from the positive output terminal of the bridge rectifier circuit Di flows to the smoothing capacitor Ci via the path from the switching diode D1 to the high-frequency choke coil L10 (from L10A to L10B). At this time, the switching output current obtained in the primary winding N1 is regenerated as power for the choke coil winding portions L10A and L10B coupled to the switching output feedback node, and is fed back to the smoothing capacitor Ci via the high-frequency choke coil L10. Since the choke coil winding portions L10A and L10B arise from the division of one inductor due to the formation of a tap, the choke coil winding portions L10A and L10B can be regarded as being tightly coupled to each other (coupling coefficient is 1) in terms of magnetic coupling therebetween. Accordingly, the operation of feeding back the switching output to the power factor improvement circuit 12 can be regarded as including voltage feedback operation. Specifically, in this voltage feedback operation, an alternating voltage dependent upon the switching output transmitted to the primary winding N1 is induced in one choke coil winding portion (L10B) as a voltage, and the alternating voltage is also induced in the other choke coil winding portion (L10A) via tight coupling between the choke coil winding portions L10A and L10B. In this manner, the power factor improvement circuit 12 employs a switching output feedback system in which a power regenerative system based on magnetic coupling is combined with a voltage feedback system based on magnetic coupling. In response to this switching output feedback, the switching diode D1 operates to implement switching for the rectified current to thereby achieve power factor improvement similarly to the above-described embodiments.
Also in a power factor improvement circuit 13 shown in
According to this connecting structure, a series circuit of the switching diode D1 and the high-frequency winding portion L10A is provided in the middle of the rectified current line between the positive output terminal of a bridge rectifier circuit Di and the positive electrode of the smoothing capacitor Ci. Furthermore, the primary winding N1 is coupled via the inductance of the high-frequency winding portion L10B to the connecting node (tap) between the choke coil winding portions L10A and L10B, which is the switching output feedback node.
The thus constructed power factor improvement circuit 13 provides operation in which the switching output current obtained in the primary winding N1 is regenerated as power and is fed back to the smoothing capacitor Ci via the high-frequency winding portion L10B and the high-frequency winding portion L10A. That is, switching output feedback due to a power regenerative system based on magnetic coupling is carried out. In addition, the power factor improvement circuit 13 can be regarded as also providing voltage feedback operation. Specifically, an alternating voltage dependent upon the switching output transmitted to the primary winding N1 is induced in the choke coil winding portion L10B as a voltage, and the alternating voltage is also induced in the choke coil winding portion L10A via tight coupling between the choke coil winding portions L10A and L10B. In this manner, the power factor improvement circuit 13 also employs a switching output feedback system in which a power regenerative system based on magnetic coupling is combined with a voltage feedback system based on magnetic coupling. The switching diode D1 implements switching for the rectified current in response to the switching output fed back through this operation, and thus power factor improvement is achieved.
If the switching output is fed back via the primary winding N1 based on a configuration in which the high-frequency choke coil L10 is provided with a tap like the above-described third and fourth embodiments, the power conversion efficiency when the load is heavy is enhanced. This is due to the power factor improvement operation in which power regeneration and voltage feedback arise in a complex manner. This complex operation of the power regeneration and voltage feedback increases the feedback amount of the switching output. Therefore, for obtaining the requisite power factor, such power supply circuit design is available that the amount of power used in the primary winding N1 is reduced corresponding to the increase of the switching output feedback amount. This leads to a decrease in power loss.
In the third and fourth embodiments, the power factor can be varied depending on the numbers of turns (turn ratio) of the high-frequency winding portions L10A and L10B. It has been confirmed that the larger number of turns of the high-frequency winding portion L10B allows a higher power factor but causes corresponding power loss. In the present embodiment, in terms of balance between the necessary power factor and power loss, the numbers of turns of the high-frequency winding portions L10A and L10B are set so that the relationship L10A>L10B is satisfied.
The third and fourth embodiments can employ the feature of the second embodiment. Specifically, the power factor improvement circuits 12 and 13 of the third and fourth embodiments may be provided with a slow-recovery rectifier diode D1A so that the rectified current from the bridge rectifier circuit Di is branched into the path including the switching diode D1 and into the path of the rectifier diode D1A. In both the third and fourth embodiments (the power factor improvement circuit 12 (
Parameters of main components are as follows: an isolation converter transformer PIT employs an EER-35 core and has a gap G of 2.2 mm, the numbers of turns of primary and secondary windings N1 and N2 are 36 T and 30 T, respectively, a coupling coefficient k is 0.67, the capacitance of a primary-side parallel resonant capacitor Cr is 6800 pF, and the capacitance of a secondary-side series resonant capacitor C2 is 0.047 μF.
In a power factor improvement circuit 14 in
The fifth embodiment includes, besides the components of the first embodiment, the following circuits in order to control the variable high-frequency choke coil PRT: a circuit for detecting a signal SDi corresponding to the current flowing through a bridge rectifier circuit Di; a circuit for detecting a signal SEi corresponding to the magnitude of a rectified and smoothed voltage Ei; and a power factor control circuit 3 that controls the magnitude of the inductance of the inductor L10 arising in the high-frequency choke coil winding N1O of the variable high-frequency choke coil PRT depending on the magnitude of the signal SDi and/or the magnitude of the signal SEi.
The detecting of the signal SDi corresponding to the current flowing through the bridge rectifier circuit Di is implemented through detecting of the voltage across a resistor R51. The detecting of the signal SEi corresponding to the magnitude of the rectified and smoothed voltage Ei is implemented through division of the rectified and smoothed voltage Ei with use of resistors R52 and R53.
The power factor control circuit 3 implements control as follows. The power factor control circuit 3 applies a current to the PFC control winding Nc to thereby control the magnitude of the inductance of the inductor L10 in the variable high-frequency choke coil PRT. This current is based on the signal SEi, which depends on the magnitude of the rectified and smoothed voltage Ei, and/or the signal SDi, which depends on the magnitude of absolute value of the current flowing from a commercial alternating-current power supply AC to the bridge rectifier circuit Di. Specifically, the magnitude of the current flowing to the PFC control winding Nc ranges from 50 mA (milliampere) to 5 mA, and correspondingly the inductance of the inductor L10 varies in the range from 10 μH (microhenry) to 50 μH.
In this manner, the current based on the signal SDi and/or the signal SEi is applied to the PFC control winding Nc to thereby control the magnitude of the inductance of the inductor L10 in the variable high-frequency choke coil PRT, so that the magnitude of the parallel resonant current can be changed. That is, the power factor can be improved favorably by controlling the magnitude of the inductance of the inductor L10 depending on changes of the load power and the AC input voltage VAC.
In addition,
As is apparent from
The control circuit 3 may use either one of the signals SDi and SEi for control.
By thus obtaining from different windings the current to flow through the variable high-frequency choke coil L10 and the current to flow through the switching diode D1, the power factor can be improved and the flexibility in circuit design can be enhanced.
Descriptions will be made below with reference to
In order to form a power factor improvement circuit 16 in the power supply circuit of
In addition, in the power factor improvement circuit 16, the anode of a fast-recovery switching diode D1 (power factor improvement switching element) is connected to the positive output terminal of a bridge rectifier circuit Di. The cathode of the switching diode D1 is connected in series to a high-frequency choke coil L10, and is coupled to the winding-finish end of the tertiary winding N3 via the choke coil L10. The tap for the primary-side winding as the connecting node between the winding-start end of the tertiary winding N3 and the primary winding N1 is connected to the positive electrode of a smoothing capacitor Ci. That is, a series circuit of the switching diode D1 (anode to cathode), the high-frequency choke coil L10 and the tertiary winding N3 is provided between the positive output terminal of the bridge rectifier circuit Di and the positive electrode of the smoothing capacitor Ci.
Furthermore, a filter capacitor CN is connected in parallel to the series circuit of the switching diode D1, the high-frequency choke coil L10 and the tertiary winding N3. The filter capacitor CN is to suppress normal mode noise.
The operation of the thus formed power factor improvement circuit 16 is as follows.
The primary-side switching converter in the power supply circuit of
The tertiary winding N3 is provided together with the switching diode D1 and the high-frequency choke coil L10 in the middle of the rectified current path through which the rectified current flows from the bridge rectifier circuit Di to the smoothing capacitor Ci. Therefore, the alternating voltage induced in the tertiary winding N3 is superimposed on the rectified output voltage from the bridge rectifier circuit Di. That is, this power supply circuit employs a voltage feedback system in which the switching output of the switching element Q1 is fed back as a voltage to the rectified current path via the magnetic coupling between the primary and tertiary windings N1 and N3.
Since the feedback voltage is thus superimposed on the rectified current path, the voltage applied to the switching diode D1 has the waveform resulting from superposition of the alternating voltage component with the switching cycle on a rectified and smoothed voltage Ei. Due to application of this alternating voltage component superimposed on the rectified and smoothed voltage Ei to the fast-recovery switching diode D1, the diode D1 implements switching operation when the absolute value of positive/negative level of the AC input voltage VAC is larger than about half of its peak value for example, to thereby intermittently block the rectified current directed toward the smoothing capacitor Ci.
The conduction period of envelope of the thus flowing rectified current is such that the current flows also during the periods when the level of the rectified output voltage from the bridge rectifier circuit Di is lower than that of the voltage across the smoothing capacitor Ci. In addition, the conduction period of the AC input current IAC that flows based on the AC input voltage VAC substantially corresponds with the conduction period of this rectified current. Specifically, the conduction angle of the AC input current IAC is increased compared with a configuration including no power factor improvement circuit, and the waveform of the AC input current IAC is brought close to that of the AC input voltage VAC. That is, power factor improvement is achieved.
For an experiment on an actual power supply circuit with the above-described configuration of
The isolation converter transformer PIT was based on the structure shown in
The number of turns of the tertiary winding N3 was set to 9 T.
For the high-frequency choke coil L10, an EE-22 core was chosen and a gap of 1.4 mm was formed, so that the inductance thereof was set to 27 μH.
As is well known, the above-described EER and EE cores are one of types and standards of product cores. It is known that the types also include an ER type. The sectional shapes of cores of all the EER, ER and EE types are an E-character shape or EE-character shape. Therefore, cores of all the EER, ER and EE types are treated as the E-shaped core and EE-shaped core in the present specification.
The capacitances of the primary-side parallel resonant capacitor Cr, the filter capacitor CN and the secondary-side series resonant capacitor C2 were chosen as follows: Cr=7500 pF, CN=1 μF and C2=0.047 μF.
The allowable load power range was from the maximum load power Pomax of 300 W to the minimum load power Pomin of 0 W (no load). The rated level of the secondary-side DC output voltage Eo was 175 V.
As the experimental results on the power supply circuit in
The power factor PF, which depends on the operation of the power factor improvement circuit 16, has a tendency of increasing as the load becomes lighter for the load variation range from the maximum load power Pomax of 300 W to the load power Po of about 100 W. In contrast, the power factor PF tends to decrease as the load becomes lighter in the range of the load power Po smaller than 100 W. As a whole, the power factor PF is higher than 0.75 for the variation range of the load power Po from 300 W to 25 W, and therefore it can be said that the power factor sufficient for practical use is obtained. For variation of the AC input voltage VAC, the power factor PF shows a characteristic of decreasing with a gentle slope as the AC input voltage VAC becomes higher.
The reason for the above-described tendency that the level of the power factor PF increases as the load becomes lighter is because constant-voltage control is based on PWM control. Specifically, in response to an increase in the level of the secondary-side DC output voltage Eo associated with a decrease in the load, the OFF period of the switching element Q1 within one switching cycle is extended in the driving of the switching element Q1. Thus, the ON period is correspondingly shortened, which raises the peak level of the resonant pulse voltage (appearing as a voltage V1) arising due to the charging/discharging of a current to/from the primary-side parallel resonant capacitor Cr during the ON period. In this manner, when the load becomes lighter, the resonant pulse voltage with an increased peak level is fed back, which increases the voltage feedback amount. This increase in the voltage feedback amount enhances the power factor, which is improved by the power factor improvement circuit 16 employing a voltage feedback system. In addition, the increase in the feedback amount also enhances the level of the voltage superimposed on the smoothing capacitor Ci, which also raises the level of the rectified and smoothed voltage Ei.
As for the AC to DC power conversion efficiency (ηAC→DC), it is almost constant against load variation, while for AC input voltage variation it increases with a gentle slope as the AC input voltage VAC becomes higher. As a result of the measurement, the efficiency ηAC→DC was 90.8% under the condition of the maximum load power Pomax of 300 W and the AC input voltage VAC of 100 V. As a comparison, the AC to DC power conversion efficiency of the power supply circuit in
A power factor improvement circuit 17 in the power supply circuit shown in
The power factor improvement circuit 17 is provided in the middle of the rectified current path in a rectifying and smoothing circuit for producing a DC input voltage (Ei) from a commercial alternating-current power supply AC, and employs a configuration for achieving power factor improvement by use of a voltage feedback system.
As shown in the drawing, in the power factor improvement circuit 17, the anode of a fast-recovery switching diode (power factor improvement switching element) D1 is connected to the positive output terminal of a bridge rectifier circuit Di. The cathode of the switching diode D1 is connected in series to a secondary winding N12 of a power factor improvement transformer VFT, and is coupled via the secondary winding N12 to the positive electrode of a smoothing capacitor Ci. That is, in the rectified current path for producing the rectified and smoothed voltage Ei, a series circuit of the switching diode D1 and the secondary winding N12 is provided in the middle of the line between the positive output terminal of the bridge rectifier circuit Di and the positive electrode of the smoothing capacitor Ci. The secondary winding N12 serves as an inductance, i.e., a choke coil against the rectified current that is an alternating current intermittently flowing with the switching cycle.
A filter capacitor CN is provided to absorb an alternating component with the switching cycle arising due to the switching operation of the switching diode D1 to thereby suppress normal mode noise. In this circuit, the filter capacitor CN is connected in parallel to the series circuit of the switching diode D1 and the secondary winding N12.
The power factor improvement transformer VFT has a structure in which a primary winding (power factor improvement primary winding) N11 and the secondary winding (power factor improvement secondary winding) N12 are wound around a core so as to be magnetically coupled to each other. The power factor improvement transformer VFT includes a so-called split bobbin in which divided winding positions are defined. The primary winding N11 and the secondary winding N12 are wound around the split bobbin and in the respective winding positions separately. This structure provides a certain coupling coefficient corresponding to loose coupling as the coupling degree between the primary and secondary sides.
In the thus constructed power factor improvement circuit 17, current and voltage as the switching output obtained in a primary winding N1 of an isolation converter transformer PIT are transmitted to the primary winding N11 in the power factor improvement transformer VFT, which is provided in series between the primary winding N1 and the smoothing capacitor Ci. Thus, a voltage with the switching cycle is generated in the primary winding N11 of the power factor improvement transformer VFT. In the power factor improvement transformer VFT, the alternating voltage obtained in the primary winding N11 induces an alternating voltage in the secondary winding N12.
The secondary winding N12 in the power factor improvement transformer VFT is connected in series to the switching diode D1, and is provided in the middle of the path through which the rectified current arising from rectification of a commercial alternating-current supply voltage AC flows.
Therefore, the alternating voltage induced in the secondary winding N12 is superimposed on the rectified output voltage.
Specifically, due to the power factor improvement transformer VFT, the primary-side series resonant current is fed back as a voltage to the smoothing capacitor Ci existing in the rectified current path via the magnetic coupling in the power factor improvement transformer VFT. This power factor improvement circuit system that feeds back the switching output as a voltage to thereby improve the power factor is referred to as a voltage feedback system.
The switching diode D1 operates to implement switching for (intermittently block) the rectified current due to the above-described superimposed alternating voltage. As a result of this operation, the rectified current flows also during the periods when the level of the AC input voltage VAC is lower than that of the voltage across the smoothing capacitor Ci. That is, the conduction angle of the rectified current is increased.
In step with this increase in the conduction angle of the rectified current, the conduction angle of the AC input current IAC is also increased. Thus, the average waveform of the AC input current IAC approaches the waveform of the AC input voltage VAC, which leads to power factor improvement.
For an experiment on an actual power supply circuit with the above-described configuration of
The isolation converter transformer PIT was based on the structure shown in
For the power factor improvement transformer VFT, an EE-25 core was chosen and a gap of 1.6 mm was formed. In addition, the primary and secondary windings N11 and N12 with the predetermined numbers of turns were wound, so that inductances L11 and L12 of the primary and secondary windings N11 and N12 were set to 60 μH and 27 μH, respectively.
As is well known, the above-described EER and EE cores are one of types and standards of product cores. It is known that the types also include an ER type. The sectional shapes of cores of all the EER, ER and EE types are an E-character shape or EE-character shape. Therefore, cores of all the EER, ER and EE types are treated as the E-shaped core and EE-shaped core in the present specification.
The capacitances of the primary-side parallel resonant capacitor Cr, the filter capacitor CN and the secondary-side series resonant capacitor C2 were chosen as follows: Cr=0.01 μF, CN=1 μF and C2=0.047 μF.
The allowable load power range was from the maximum load power Pomax of 300 W to the minimum load power Pomin of 0 W (no load). The rated level of the secondary-side DC output voltage Eo was 175 V.
As the experimental results on the power supply circuit in
The power factor PF, which depends on the operation of the power factor improvement circuit 17, has a tendency of decreasing in the range of about 0.8 to 0.75 as the load becomes lighter for the load variation range from the maximum load power Pomax of 300 W to about 100 W. In contrast, the power factor PF tends to increase as the load becomes lighter in the range of the load power Po smaller than 100 W. As a whole, the power factor PF is higher than 0.75 for the variation range of the load power Po from 300 W to 25 W, and therefore it can be said that the power factor sufficient for practical use is obtained. For variation of the AC input voltage VAC, the power factor PF shows a characteristic of being almost constant at about 0.8 for the range of the voltage VAC from 85 V to 140 V.
The reason for the above-described tendency that the level of the power factor PF increases as the load power Po becomes lower from about 100 W is that constant-voltage control is based on PWM control. Specifically, in response to an increase in the level of the secondary-side DC output voltage Eo associated with a decrease in the load, the OFF period of a switching element Q1 within one switching cycle is extended in the driving of the switching element Q1. Thus, the ON period is correspondingly shortened, which raises the peak level of the resonant pulse voltage (appearing as a voltage V1) arising due to the charging/discharging of a current to/from the primary-side parallel resonant capacitor Cr during the ON period. In this manner, when the load becomes lighter, the resonant pulse voltage with an increased peak level is fed back, which increases the voltage feedback amount. This increase in the voltage feedback amount enhances the power factor, which is improved by the power factor improvement circuit 17 employing a voltage feedback system. In addition, the increase in the feedback amount also enhances the level of the voltage superimposed on the smoothing capacitor Ci, which also raises the level of the rectified and smoothed voltage Ei.
As for the AC to DC power conversion efficiency (ηAC→DC), it is almost constant against load variation, while for AC input voltage variation it increases with a gentle slope as the AC input voltage VAC becomes higher. As a result of the measurement, the efficiency ηAC→DC was 91.1% under the condition of the maximum load power Pomax of 300 W and the AC input voltage VAC of 100 V. As a comparison, the AC to DC power conversion efficiency of the power supply circuit in
It should be noted that the present invention is not limited to the above-described configurations as the embodiments. For example, other configurations are also available as the circuit configuration of details of the primary-side voltage resonant converter, and the configuration of the secondary-side rectifier circuit including a secondary-side series resonant circuit.
In addition, as the switching element, e.g. an IGBT (Insulated Gate Bipolar Transistor) or a bipolar transistor may be used instead of a MOS-FET. Moreover, although the above-described embodiments employ a separately-excited switching converter, the present invention can also be applied to a configuration employing a self-excited switching converter.
It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.
Claims
1. A switching power supply circuit, comprising:
- a rectifier operable to perform a rectifying operation on a supplied alternating-current voltage to produce a rectified voltage;
- a smoothing unit including a smoothing capacitor operable to smooth the rectified voltage;
- a switching unit including a switching element operable to input the smoothed rectified voltage as a direct-current input voltage and to perform a switching operation;
- a switching drive unit operable to drive the switching element to perform the switching operation;
- an isolation converter transformer having a primary winding on a primary side and a secondary winding on a secondary side, the primary winding being supplied with a switching output obtained from the switching operation of the switching element, and the secondary winding having an alternating voltage induced therein by the primary winding, the isolation converter transformer having a structure producing a coupling coefficient between the primary side and the secondary side such that adequate zero voltage switching is ensured in an intermediate load condition;
- a primary-side parallel resonant circuit including a leakage inductance component of the primary winding and a capacitance of a primary-side parallel resonant capacitor, the primary-side parallel resonant circuit making the switching unit operate on a voltage resonant basis;
- a secondary-side series resonant circuit including a leakage inductance component of the secondary winding and a capacitance of a secondary-side series resonant capacitor connected in series with the secondary winding;
- a secondary-side direct-current output voltage producing unit operable to input the alternating voltage induced in the secondary winding of the isolation converter transformer, and. to perform a rectifying operation on the input alternating voltage to produce a secondary-side direct-current output voltage;
- a controller operable to supply the switching drive unit with a detection output dependent upon a level of the secondary-side direct-current output voltage to thereby stabilize the level of the secondary-side direct-current output voltage; and
- a power factor improving unit including a power factor improvement switching element operable to intermittently block a rectified current due to superposition of the switching output on the rectified output of the rectifier, the superposition being attributed to feedback of the switching output obtained in the primary winding to the smoothing capacitor.
2. The switching power supply circuit according to claim 1, wherein the power factor improving unit includes a high-frequency choke coil forming a series circuit with the power factor improvement switching element, the series circuit being provided between the rectifier and the smoothing unit.
3. The switching power supply circuit according to claim 2, wherein one end of the primary winding of the isolation converter transformer is connected to the primary-side parallel resonant capacitor and the switching unit that are connected in parallel to each other, and the other end of the primary winding is coupled to a connecting node between the high-frequency choke coil and the power factor improvement switching element, so that a switching output current as the switching output obtained in the primary winding is regenerated for the smoothing capacitor via the high-frequency choke coil.
4. The switching power supply circuit according to claim 3, wherein the power factor improving unit further includes a filter capacitor provided in parallel to the series circuit, and a rectifying element connected in parallel to the filter capacitor and allowing branching of a rectified current obtained from the rectifying operation of the rectifier into the smoothing capacitor.
5. The switching power supply circuit according to claim 3, wherein the high-frequency choke coil is a variable high-frequency choke coil formed by winding around a saturable core a power factor improvement control winding and a high-frequency choke coil winding for applying the switching output current obtained in the primary winding to the smoothing capacitor, and a value of an inductance arising in the high-frequency choke coil winding is controlled by applying to the power factor improvement control winding a current that depends on a magnitude of the rectified and smoothed voltage and/or a magnitude of an absolute value of a current flowing to the rectifier due to input of the alternating-current voltage.
6. The switching power supply circuit according to claim 2, wherein the isolation converter transformer includes a tertiary winding that boosts a switching output voltage as the switching output, and the series circuit formed by the power factor improvement switching element and the high-frequency choke coil is formed via the tertiary winding.
7. The switching power supply circuit according to claim 6, wherein
- the isolation converter transformer includes a tertiary winding that boosts a switching output voltage as the switching output, and the series circuit formed by the power factor improvement switching element and the high-frequency choke coil is formed via the tertiary winding, and
- the switching output voltage boosted by the tertiary winding is fed back to the smoothing capacitor so that the switching output voltage is superimposed on the rectified voltage produced by the rectifier.
8. The switching power supply circuit according to claim 2, wherein
- the power factor improving unit includes a power factor improvement transformer having a primary winding and a secondary winding loosely coupled to each other, the power factor improvement transformer feeding back a switching output voltage obtained in the primary winding of the isolation converter transformer,
- one end of the primary winding of the isolation converter transformer is connected to the primary-side parallel resonant capacitor and the switching unit that are connected in parallel to each other, and the other end of the primary winding of the isolation converter transformer is coupled to the smoothing capacitor via the primary winding of the power factor improvement transformer, and
- the secondary winding of the power factor improvement transformer serves as the high-frequency choke coil.
Type: Application
Filed: Jun 30, 2006
Publication Date: Feb 15, 2007
Applicant: Sony Corporation (Tokyo)
Inventor: Masayuki Yasumura (Kanagawa)
Application Number: 11/479,734
International Classification: H02M 3/335 (20060101);